There are achievements, which you will get after finishing an Episode. (Each Episode got one achievement.)

Don’t Leave me Hanging
Complete the First Episode.


A Secret from the Past
Complete the Second Episode.


The Truth Behind the Curtain
Complete the Third Episode.


The Veil
Complete the Fourth Episode.

The Other Story
Complete all minisodes.


In the Extras-Menu of the game you will find the four minisodes:
- The Morgue
- Death Sentence
- Wanderer in the Fog
- Beechworth's Wake

In order to unlock the achievement you have to finish all of these minisodes.

Better Served Silent
End the poor bird's suffering.


In the first chapter you have to get the crow.











After that you have to get the hammer.











Now use the hammer with the crow and you get the achievement.

Art Restoration
Reveal the picture beneath.


In the first chapter get the cloth from the kitchen.










After that get the paint thinner.










Then combine the cloth with the paint thiner.










Finally use the cloth with paint thinner on the picture and you should get the achievement.

The Departed
Find something left behind.


First you have to use the music box in this room with the nun.










After that go to the beach and you will find cloths of a nun. Just look at them and you should get the achievement.

Merciful
Ease the pain of a dying man.


After you distracted the nun from this man, he asks you to help him dying. Just take the second option. Otherwise you won't be able to get the achievement.










Now you should go back to the room, where you used the music box and get the morphine there.










Later, when you get into the basement, use the morphine on the poor man and you should get the achievement.

Full Deck
Uncover all five tarot cards.


For this achievement you will need at least two runs to the fortuneteller in Episode Three.

After it turned night, you will find a fortuneteller in the streets. Ask her to help you and tell her you have no penny for paying her.




















Now you have the option to pick three cards. After you picked them restart the Episode and get to the fortuneteller again and pick three cards again. You have to do that until you have seen five different cards and then the achievement should be yours.

The Short Path
Navigate through the fog like a pro.


Towards the end of Episode Three you have to navigate through fog, just get through it without failing. (Correct order of noises: Wind, Birds, Sea, Silence)
The screen you should wander through are the following (the order you visit them is random as well as the sounds in it, so you have to listen carefully to find the door with the right sound (Thanks to Fraudaline and 5M0k1N for pointing that out)) :








































If you arrive on the following screen and have done everything correct, you should get the achievement.

Alchemist
Become an expert developer.


To get this achievement, you have to develop the photo without failing.

First you have to use vitrol on the photo.










After that you have to use cyanide on it.










Finally use water on it and you should get the achievement, if you haven't made any mistake.

The Angels Protect this House
Fail to escape your destiny.


After you finished the telescope puzzle (IMPORTANT: You need the crystal, which is required to solve it. Thanks to Spirit Lost), go back to the room, where you came from, and you should get the achievement.
Edit: You just need the crystal and then you can go back in the previous room and the angels should be blocking your way.

The Whole Story
Read all letters, notes and diaries in the game.


Mentioned by Absurd Logic:
"The Whole Story" achievement is a delayed one. Unlike "Master of the Occult" which unlocks immediately, the last note by the telescope doesn't trigger anything that the player can see. It doesn't pop till after you finish the final scene AND go back to the Main Menu (not the "Extras" screen after the credits, but the very first menu where you continue or choose episodes). It has nothing to do with the credits. I skipped them and still got it.

Letters/Notes/Diaries in Episode One "The Letter":

The first letter you will is in the entrance hall of the house.










Another one you will in the room with the bed, where you later have to put the crow in the pan for the cat.










Four letters/notes/diares are found in the room with the dead woman.
Three are on the ground and one you will find at the dead woman.



















In the room with the hammer and paint thinner you will another two.










The last one of Episode One is found at the hanged Anthony.












Letters/Notes/Diaries in Episode Two "Memories":

There are two letters found in the old classroom.










A note is found on the bed next to the statue, after the man disappeared.










After Mother Elizabeth went outside to the person, who fell out of the coffin, you will find one on the reception desk.










There is also a note on the ground in the room of Father Ernest.










Many notes/letter are found on the ground, while you are wandering around in your dream.

























































Letters/Notes/Diaries in Episode Three "The Four Witnesses":

The first note/letter you will find next to the girl sitting on the bed.










The next note you will find in the first room of the bookstore.










Another note in the bookstore you will find in the same room, where the tree is located.










The next note is located in the cellar below the tree next to its roots.










The last note of the Episode you shouldn't miss, because it's the note, which will help you navigate through the fog. It's next to the stairs, which will lead to the place, where the fortuneteller sits.












Letters/Notes/Diaries in Episode Four "Ancient Shadows":

The first letter is found in the same room, where you find the lamp.










The next note, you will find one Alexandre's bed.










In the chest in front of the dark room is another letter/note.










The dark room itself contains also a note, which will help you with developing the photo.










Another note is found on the desk in Alexandre's office.










The following note you will find in the cellar on a desk.










After you dig up the coffin, you will find a note next to the skeleton.










After Alexandre disappeared you will find a note/letter on his chair.










The last and final note/letter you will find next to the telescope.

Explorer of the Occult
Discover your first secret.


Just find one of the secrets listed below and you will get the achievement.

Master of the Occult
Discover all secrets in the game.


Secrets in Episode One "The Letter":

None.


Secrets in Episode Two "Memories":

At the location of the second letter in the classroom, you will also find the first secret. Just stand there for a while and you will hear some strange noises. That's the secret. (Thanks at the developer for correcting my information about the secret.)










Some people say that you get the following secret only after finding all others, but I got it before I had all other secret.
Also this guide most likely will help you, if you have problems with it:
http://steamproxy-script.pipiskins.com/sharedfiles/filedetails/?id=362543090
Before you use the music box in the room with the nun, talk to her and take the first option. After that use the music box.










After that go to the beach, the nun will be there. Go and talk to her.











Secrets in Episode Three "The Four Witnesses":

After it turned night, go back from where you came at the beginning of the episode. There will now be a painted red big eye on the wall. That's the secret.










For the next secret don't go further than the picture below shows you, because if the man goes into the house the secret is lost. (you have to restart the episode for another try)










You have to light the latern first. After that come back follow the man into the house and go straight to the garden and you will see the man turning into crows.










There is also a secret in the bookstore. Next to the men there is a bookshelf, where you have to find five different books.
Their titles are:
-The Horola by Guy de Maupassant
-The Great God Pan by Arthur Machen
-The Silver Key by Howard Phillips Lovecraft
-The Approach to Al-Mu'tasim by Jorge Luis Borges
-The Black Cat by Edgar Allan Poe





























Another secret is found in the room with the gramophone, just turn it off and on until you hear something different than laughing or crying.











Secrets in Episode Four "Ancient Shadows":

The first secret is quite right at the beginning of the chapter. Just go in the greenhouse without lantern and wait for the sound of breaking glass.










The last secret is located after Alexandre's scream. Just go in his room and after the windows open wait there.










After some time a crow will appear and says "Nevermore.".

Thanks to all people, who helped finishing the guide, especially the people, who helped with the missing secret and especially Pillow Master for his guide containing the secret.
Also a big thanks to Fernandoloco™ for helping me correcting some mistakes in my guide and keeping me informed about new discovered secrets. :)

If somebody finds any mistake, write it in the comments and I will correct it as soon as possible.